The present invention relates to a method and apparatus for combining a magnetohydrodynamic generator and vapor generator to utilize the product gas and heat output of a coal gasifier to generate power. More specifically, the char and a first portion of the low BTU product gas produced in an atmospheric pressure coal gasifier are used to fuel a MHD burner, while a second portion of the low BTU product gas is used as an independent source of fuel to preheat combustion air for the MHD burner.
In a magnetohydrodynamic (MHD) generator, fuel and air are combusted to form a high temperature working fluid. The working fluid must be formed at a sufficiently high temperature, at least 4000 F., to promote ionization therein. The working fluid thus formed is passed through a static magnetic field at a high velocity thereby inducing direct current. Ergo, the MHD process provides for the direct conversion of energy into electricity without the need of a conventional turbine generator. When combined with a vapor generator disposed downstream of the MHD generator, wherein the waste heat contained in the gas products produced in the MHD burner is utilized to generate steam as an auxiliary source of power, overall efficiencies in the area of 50 percent can be obtained.
When integrating a MHD generator with a vapor generator, a number of problems have been encountered to which the present invention is addressed. First, present day technology dictates the use of an independently-fired air heater to provide high temperature, approximately 3000 F., combustion air to the MHD burner. Since the independently-fired air heater must utilize a clean fuel in order to prevent fouling and corrosion of heat transfer surface immersed therein, oil or gas is required. Coal, because of its high ash content, cannot serve directly as the fuel for an independently-fired air heater.
Second, economics require that the seed material introduced into the MHD burner to enhance the electrical conductivity of the working fluid discharged into the MHD channel must be reclaimed and recycled. Unfortunately, the high sulfur content of most coals proposed as fuel for the MHD burner results in sulfur contamination of the seed. Thus, expensive reprocessing of the reclaimed seed is required in order to remove the sulfur contamination.
Third, the temperature of the working fluid must be sufficiently high, i.e., in the range of 4000 to 5000 F., in order to insure ionization and proper electrical conductivity. Thus, it is preferred that the fuel fired in the MHD burner have a low moisture content and a low hydrogen content so that the amount of available energy lost to the latent heat of vaporization of water delivered to or formed in the combustion process is minimized.